Alkali cell

ABSTRACT

An alkaline battery comprising: a positive electrode mixture comprising manganese dioxide and nickel oxyhydroxide as active materials; a negative electrode comprising zinc as an active material; and an alkaline electrolyte, characterized in that the potential of the manganese dioxide relative to a mercury/mercury oxide electrode in a potassium hydroxide aqueous solution having a KOH concentration of 40 wt % is 270 mV or higher.

TECHNICAL FIELD

The present invention relates to an improvement of an alkaline batterythat utilizes manganese dioxide and nickel oxyhydroxide as positiveelectrode active materials.

BACKGROUND ART

Alkaline batteries such as alkaline dry batteries comprise, for example,a positive electrode case which also serves as a positive electrodeterminal, a cylindrical positive electrode mixture which is closelyfitted to the inner face of the case, and a gelled negative electrodewhich is disposed in a hollow space of the positive electrode mixturewith a separator interposed therebetween.

With a recent increase in load of equipment in which alkaline batteriesare used, there is an increasing demand for alkaline batteries havingexcellent heavy load discharge performance. Thus, mixing nickeloxyhydroxide into the positive electrode mixture has been examined toimprove heavy load discharge performance of alkaline batteries (e.g.,Japanese Laid-Open Patent Publication No. 2001-15106).

However, alkaline batteries including manganese dioxide and nickeloxyhydroxide as positive electrode active materials have inferiorstorage performance to alkaline batteries including no nickeloxyhydroxide, and have large self-discharge especially when stored athigh temperatures. Thus, after a long-term storage, the alkalinebatteries including nickel oxyhydroxide have such a problem that theheavy load discharge performance thereof is inferior to that of thealkaline batteries including no nickel oxyhydroxide.

The normal electrode potential of nickel oxyhydroxide is 0.49 V relativeto a normal hydrogen electrode (NHE (25° C.)), and the normal electrodepotential of manganese dioxide is 0.15 V relative to an NHE (25° C.).

Also, the potential of nickel oxyhydroxide in a KOH aqueous solution(KOH concentration 40 wt %) is 370 to 410 mV relative to an Hg/HgOelectrode, and the potential of electrolytic manganese dioxide in a KOHaqueous solution (KOH concentration 40 wt %) is, for example, 240 to 270mV relative to an Hg/HgO electrode. Electrolytic manganese dioxidehaving such a potential is generally used in conventional alkalinebatteries including no nickel oxyhydroxide (Japanese Laid-Open PatentPublication No. Hei 7-183032).

The potential of manganese dioxide is designed in consideration ofstorage characteristics of batteries. When the potential of manganesedioxide is high, the potential difference between a positive electrodeand a negative electrode including zinc as an active material becomeslarge. This allows the oxidation reaction of manganese dioxide toproceed readily even while the battery is not used, thereby invitingdeterioration in storage characteristics. Meanwhile, when the potentialdifference between the negative electrode and the positive electrodebecomes large, the open circuit voltage of the battery becomes high.However, a major improvement in voltage maintaining characteristics andhigh rate discharge characteristics is not observed. Therefore,manganese dioxide having a potential of approximately 250 mV which isnot too high and not too low has become predominant.

It is known that the potential of manganese dioxide is changed, forexample, by controlling the conditions of electrolysis which isperformed to obtain the potential (Japanese Laid-Open Patent PublicationNo. 2002-348693).

DISCLOSURE OF INVENTION

An object of the present invention is to provide an alkaline batterycapable of retaining heavy load discharge performance even after along-term storage at high temperatures.

An alkaline battery in accordance with the present invention comprises:a positive electrode mixture comprising manganese dioxide and nickeloxyhydroxide as active materials; a negative electrode comprising zincas an active material; and an alkaline electrolyte, and is characterizedin that the potential of the manganese dioxide relative to amercury/mercury oxide electrode (Hg/HgO electrode) in a potassiumhydroxide aqueous solution having a KOH concentration of 40 wt % is 270mV or higher.

With respect to the total amount of the manganese dioxide and the nickeloxyhydroxide, it is preferable that the content of the manganese dioxidebe from 20 to 90 wt % and that the content of the nickel oxyhydroxide befrom 10 to 80 wt %.

As the manganese dioxide, it is possible to use electrolytic manganesedioxide of which potential is heightened by cleaning with an aqueoussolution of sulfuric acid.

The concentration of sulfuric acid in the aqueous solution of sulfuricacid is preferably 10 wt % or higher.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially sectional front view of an example of an alkalinebattery in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

One of the reasons for the self-discharge of alkaline batteriesincluding manganese dioxide and nickel oxyhydroxide is that thepotential difference between manganese dioxide and nickel oxyhydroxidecauses formation of a local battery consisting of manganese dioxide andnickel oxyhydroxide in a positive electrode mixture in which anoxidation reduction reaction proceeds.

Thus, in order to make such batteries after storage retain heavy loaddischarge characteristics, the potential difference between manganesedioxide and nickel oxyhydroxide needs to be reduced to avoiddeterioration of nickel oxyhydroxide caused by the formation of thelocal battery. For this purpose, it is effective to heighten thepotential of manganese dioxide and bring it close to the potential ofnickel oxyhydroxide.

However, manganese dioxide having a relatively low potential ofapproximately 250 mV relative to an Hg/HgO electrode in a KOH aqueoussolution (KOH concentration 40 wt %) has conventionally been used alsoin alkaline batteries to which nickel oxyhydroxide is added to improveheavy load discharge performance. This is presumably because thepotential of manganese dioxide has been selected based on theabove-described technically common knowledge about conventional alkalinebatteries including no nickel oxyhydroxide without sufficient awarenessthat the formation of the local battery causes deterioration of nickeloxyhydroxide.

In order to suppress the deterioration of nickel oxyhydroxide due to theformation of the local battery, it is effective to use manganese dioxidehaving a potential of 270 mV or higher relative to an Hg/HgO electrodein a KOH aqueous solution (KOH concentration 40 wt %). This is becausethe use of such manganese dioxide makes it possible to reduce thepotential difference between the nickel oxyhydroxide generally used inalkaline batteries and the manganese dioxide down to a level at whichthe above-described oxidation reduction reaction is suppressed.

The manganese dioxide having a potential of 270 mV or higher relative toan Hg/HgO electrode in a KOH aqueous solution (KOH concentration 40 wt%) may be prepared, for example, by electrolyzing a solution containingdivalent manganese ions. For example, in performing electrolysis usingan acidic manganese sulfate solution, the potential of manganese dioxideis varied by controlling electrolytic potential, manganese ionconcentration, sulfuric acid concentration, current density, solutiontemperature, etc (Japanese Laid-Open Patent Publication No.2002-348693). Thus, one with ordinary skill in the art could produceelectrolytic manganese dioxide having a desired potential by properlyselecting electrolytic conditions.

Also, when manganese dioxide having low potential is cleaned withsulfuric acid, lower level manganese oxides on the surfaces of manganesedioxide particles are dissolved and removed, so that manganese dioxidehaving higher potential can be obtained.

For example, electrolytic manganese dioxide is mixed with an aqueoussolution of sulfuric acid to produce slurry. The concentration ofmanganese dioxide in the produced slurry is preferably from 100 to 300g/L. The sulfuric acid concentration of the aqueous solution of sulfuricacid used therein is preferably 5 wt % or higher and more preferably 10wt % or higher. Also, the electrolytic manganese dioxide used thereinnormally has a purity of 91 to 92% with inclusion of impurities such aslower level manganese oxides, water and sulfate.

Subsequently, the slurry is stirred for 5 to 10 hours while thetemperature thereof is maintained at 45° C. to 60° C. Thereafter,manganese dioxide is filtered out, washed with water, and neutralizedwith alkali if necessary and washed again. By these steps, the potentialof manganese dioxide is heightened.

The positive electrode mixture is prepared by mixing manganese dioxidewith heightened potential and nickel oxyhydroxide together with aconductive material such as graphite and an alkaline aqueous solution.

The average particle size of manganese dioxide is preferably from 30 to50 μm, and the average particle size of nickel oxyhydroxide ispreferably from 5 to 30 μm.

Further, in order to obtain an alkaline battery having excellentdischarge performance at initial stage and after high temperaturestorage, it is preferable that with respect to the total amount ofmanganese dioxide and nickel oxyhydroxide, the content of manganesedioxide be from 20 to 90 wt % and that the content of nickeloxyhydroxide be from 10 to 80 wt %.

Also, when the content of manganese dioxide is from 20 to 80 wt % andthe content of nickel oxyhydroxide is from 20 to 80 wt %, the dischargeperformance of the alkaline battery at initial stage is furtherimproved.

EXAMPLES

An alkaline battery of AA size produced in the following examples andcomparative examples will be described with reference to FIG. 1. Itshould be noted that FIG. 1 merely illustrates an example of thealkaline battery of the present invention and is not to be construed aslimiting the present invention.

In FIG. 1, a positive electrode case 1 is made of steel plated withnickel. A graphite coating film 2 is formed on the inner face of thepositive electrode case 1. A plurality of positive electrode mixturepellets 3 in a short cylindrical shape are inserted into the positiveelectrode case 1 so as to intimately contact the inner face of the case1.

A separator 4 is provided in the hollow space of the positive electrodemixture pellets 3, and an insulating cap 5 is provided in the centralpart of the bottom of the case 1. The separator 4 and the positiveelectrode mixture pellets 3 are impregnated with an alkalineelectrolyte.

Inside the separator 4 is charged a gelled negative electrode 6. Anegative electrode current collector 10 is inserted into the centralpart of the gelled negative electrode 6. The negative electrode currentcollector 10 is pressed into a central hole of a resin sealing plate 7,and is integrally welded, at the head thereof, to a bottom plate 8. Thebottom plate 8 also functions as a negative electrode terminal. Aninsulating washer 9 is caught by the sealing plate 7.

The opening end of the positive electrode case 1 is crimped onto theouter edge of the bottom plate 8 with the outer edge of the resinsealing plate 7 interposed therebetween. Thus, the opening of thepositive electrode case 1 is sealed. The outer surface of the positiveelectrode case 1 is covered with a jacket label 11.

Example 1

(a) Process of Heightening Potential of Manganese Dioxide

HH-PF, electrolytic manganese dioxide for alkaline batteriesmanufactured by Tosoh Corporation, was used. The physical properties ofHH-PF are shown below.

-   -   MnO₂ purity: 91% or higher    -   Average particle size obtained by a micro-track method: about 40        μm    -   pH: 3.0 to 4.0    -   Electrode potential relative to an Hg/HgO electrode in a KOH        aqueous solution (KOH concentration 40 wt %): 254 mV

This electrolytic manganese dioxide was mixed with an aqueous solutionof sulfuric acid having a sulfuric acid concentration of 5 wt % toproduce slurry of 60° C. The concentration of manganese dioxide in theslurry was 100 g/L.

Subsequently, the slurry was stirred for one hour while it wasmaintained at 60° C., and thereafter, manganese dioxide was filtered outand washed with water. Then, the washed manganese dioxide was washedwith an aqueous solution of sodium hydroxide to neutralize the remainingsulfuric acid and washed again with water.

This produced manganese dioxide “a” whose potential relative to anHg/HgO electrode in a KOH aqueous solution (KOH concentration 40 wt %)was 272 mV.

(b) Preparation of Positive Electrode Mixture

The Manganese dioxide with heightened potential, nickel oxyhydroxide(average particle size 10 μm), and graphite (average particle size 20μm) were mixed in a weight ratio of 50:50:5. Further, 1 part by weightof an alkaline electrolyte per 100 parts by weight of the total ofmanganese dioxide and nickel oxyhydroxide was added to theabove-described mixture, and the resultant mixture was stirred with amixer and granulated to have a certain particle size. Therein, thealkaline electrolyte used was an aqueous solution of potassium hydroxidehaving a KOH concentration of 40 wt %. The particles thus obtained werepressurized into a hollow cylindrical shape to mold positive electrodemixture pellets A.

(c) Preparation of Gelled Negative Electrode

100 parts by weight of zinc powder as a negative electrode activematerial, 1.5 parts by weight of sodium polyacrylate as a gelling agent,and 50 parts by weight of an alkaline electrolyte (an aqueous solutionof potassium hydroxide having a KOH concentration of 40 wt %) were mixedwith each other to produce a gelled negative electrode.

(d) Production of Alkaline Battery

A plurality of positive electrode mixture pellets A thus obtained werecharged into a positive electrode case, and the pellets A were pressedagain inside the case so as to intimately contact the inner face of thecase. Subsequently, a separator was fitted to the inner face of thehollow space of the positive electrode mixture pellets A, and aninsulating cap was provided in the central part of the bottom of thecase. Then, an alkaline electrolyte (an aqueous solution of potassiumhydroxide having a KOH concentration of 40 wt %) was injected into thecase to impregnate the separator and the positive electrode mixturepellets A. Next, the gelled negative electrode was charged into thehollow space surrounded by the separator, and a predetermined negativeelectrode current collector was inserted into the central part of thegelled negative electrode to seal the opening of the case. Thereafter,by performing a predetermined crimping operation, an alkaline battery ofAA size as illustrated in FIG. 1 was assembled. This alkaline batterywas named as battery A.

(e) Evaluation of Alkaline Battery

Batteries at the initial stage and after a 7-day storage at 60° C. werecontinuously discharged at a constant electric power of 1000 mW at 20°C., and discharge duration was measured until the battery voltagereached to a cut-off voltage of 0.9 V. Also, the ratio (%) of thedischarge duration of the battery after the storage to the dischargeduration of the battery at the initial stage was obtained.

Example 2

The potential of manganese dioxide was heightened in the same manner asin Example 1 except that the concentration of sulfuric acid in theaqueous solution of sulfuric acid was changed from 5 wt % to 10 wt %, 15wt %, 20 wt % and 30 wt %. This produced electrolytic manganese dioxide“b”, “c”, “d” and “e” whose potentials relative to an Hg/HgO electrodein a KOH aqueous solution (KOH concentration 40 wt %) were 281 mV, 288mV, 297 mV and 312 mV, respectively.

Subsequently, positive electrode mixture pellets B, C, D and E wereproduced in the same manner as in Example 1 except for the use of theelectrolytic manganese dioxide “b”, “c”, “d” and “e”.

Thereafter, batteries B, C, D and E were produced in the same manner asin Example 1 except for the use of the positive electrode mixturepellets B, C, D and E, and were evaluated in the same manner as thebattery A.

Comparative Example 1

Positive electrode mixture pellets F were produced in the same manner asin Example 1 except that HH-PF, electrolytic manganese dioxide foralkaline batteries manufactured by Tosoh Corporation, was used withoutany treatment. Then, a battery F was produced in the same manner as inExample 1 except for the use of the positive electrode mixture pellets Fand was evaluated in the same manner as the battery A.

Example 3

HH-TF, electrolytic manganese dioxide for alkaline batteriesmanufactured by Tosoh Corporation, was used. The physical properties ofHH-TF are shown below.

-   -   MnO₂ purity: 91% or higher    -   Average particle size obtained by a micro-track method: about 40        μm    -   pH: 3.0 to 4.0    -   Potential relative to an Hg/HgO electrode in a KOH aqueous        solution (KOH concentration 40 wt %): 275 mV

The potential of this electrolytic manganese dioxide was heightened inthe same manner as in Example 1, thereby producing manganese dioxide “g”whose potential relative to an Hg/HgO electrode in a KOH aqueoussolution (KOH concentration 40 wt %) was 283 mV.

Subsequently, positive electrode mixture pellets G were produced in thesame manner as in Example 1 except for the use of the electrolyticmanganese dioxide “g”.

Thereafter, a battery G was produced in the same manner as in Example 1except for the use of the positive electrode mixture pellets G and wasevaluated in the same manner as the battery A.

Table 1 shows potentials of the manganese dioxide “a” to “g” anddischarge durations of the batteries A to G. It is noted that eachdischarge duration is an average value of ten batteries which isexpressed as a relative value obtained by defining the dischargeduration at the initial stage of the battery F in Comparative Example 1as 100. TABLE 1 Electrode potential Sulfuric acid of Discharge durationBattery concentration manganese Initial After (B/A) × 100 No. (wt %)dioxide stage(A) storage(B) (%) Example 1 A 5 272 102 74 73 Example 2 B10 281 103 77 75 C 15 288 102 81 79 D 20 297 100 81 81 E 30 312  97 7981 Example 3 G 5 283 104 82 79 Comparative F 5 254 100 71 71 example 1(Cut-off voltage 0.9 V)

As is clear from Table 1, the batteries A to E, which used manganesedioxide having a potential of 270 mV or higher, had an improveddischarge performance after high temperature storage in comparison withthe battery F, which used manganese dioxide having a potential lowerthan 270 mV.

In the process of heightening the potential of electrolytic manganesedioxide, the higher the sulfuric acid concentration of the aqueoussolution of sulfuric acid, the higher the potential of the resultantmanganese dioxide. Also, the higher the sulfuric acid concentration ofthe aqueous solution of sulfuric acid, the higher the ratio of thedischarge duration of the battery after storage to the dischargeduration of the battery at the initial stage.

Also, although the reason is not yet clear, the discharge durations atthe initial stage of the batteries A to D were longer than that of thebattery F while the discharge duration of the battery E was shorter.

Next, in the following examples and comparative examples, the content ofnickel oxyhydroxide with respect to the total amount of manganesedioxide and nickel oxyhydroxide was examined.

Comparative Example 2

Positive electrode mixture pellets were produced in the same manner asin Example 1 except that HH-PF, electrolytic manganese dioxide foralkaline batteries manufactured by Tosoh Corporation, was used withoutany treatment and that the contents of manganese dioxide and nickeloxyhydroxide in the positive electrode mixture were varied as shown inTable 2, and batteries 1 to 8 were assembled. Then, the batteries 1 to 8were evaluated in the same manner as the battery A of Example 1.

Table 2 shows discharge durations of the batteries 1 to 8. It is notedthat each discharge duration is an average value of ten batteries whichis expressed as a relative value obtained by defining the dischargeduration at the initial stage of the battery 1 as 100. TABLE 2 Contentin positive electrode (part by weight) Discharge duration BatteryManganese Nickel Initial After (B/A) × 100 No. dioxide oxyhydroxideGraphite stage(A) storage(B) (%) 1 100 0 5 100 92 92 2 95 5 5 102 89 873 90 10 5 107 91 85 4 80 20 5 116 94 81 5 50 50 5 138 98 71 6 20 80 5147 90 61 7 10 90 5 157 83 53 8 0 100 5 161 79 49(Cut-off voltage 0.9 V)

Example 4

HH-PF, electrolytic manganese dioxide for alkaline batteriesmanufactured by Tosoh Corporation, was used. The potential of thiselectrolytic manganese dioxide was heightened in the same manner as inExample 1 except for the use of an aqueous solution of sulfuric acidhaving a sulfuric acid concentration of 15 wt %, thereby producingmanganese dioxide whose potential relative to an Hg/HgO electrode in aKOH aqueous solution (KOH concentration 40 wt %) was 288 mV.

Positive electrode mixture pellets were produced in the same manner asin Example 1 except that the manganese dioxide thus obtained was usedand that the contents of manganese dioxide and nickel oxyhydroxide inthe positive electrode mixture were varied as shown in Table 3, andbatteries 9 to 14 were assembled. Then, the batteries 9 to 14 wereevaluated in the same manner as the battery A of Example 1.

Table 3 shows discharge durations of the batteries 9 to 14. It is notedthat each discharge duration is an average value of ten batteries whichis expressed as a relative value obtained by defining the dischargeduration at the initial stage of the battery 1 in Comparative Example 2as 100. TABLE 3 Content in positive electrode (part by weight) Dischargeduration Battery Manganese Nickel Initial After (B/A) × 100 No. dioxideoxyhydroxide Graphite stage(A) storage(B) (%)  9 95  5 5 105  93 89 1090 10 5 111 101 91 11 80 20 5 120 104 87 12 50 50 5 141 111 79 13 20 805 148 101 68 14 10 90 5 156  86 55(Cut-off voltage 0.9 V)

Example 5

HH-TF, electrolytic manganese dioxide for alkaline batteriesmanufactured by Tosoh Corporation, was used. The potential of thiselectrolytic manganese dioxide was heightened in the same manner as inExample 1, thereby producing manganese dioxide whose potential relativeto an Hg/HgO electrode in a KOH aqueous solution (KOH concentration 40wt %) was 283 mV.

Positive electrode mixture pellets were produced in the same manner asin Example 1 except that the manganese dioxide thus obtained was usedand that the contents of manganese dioxide and nickel oxyhydroxide inthe positive electrode mixture were varied as shown in Table 4, andbatteries 15 to 20 were assembled. Then, the batteries 15 to 20 wereevaluated in the same manner as the battery A of Example 1.

Table 4 shows discharge durations of the batteries 15 to 20. It is notedthat each discharge duration is an average value of ten batteries whichis expressed as a relative value obtained by defining the dischargeduration at the initial stage of the battery 1 in Comparative Example 2as 100. TABLE 4 Content in positive electrode (part by weight) Dischargeduration Battery Manganese Nickel Initial After (B/A) × 100 No. dioxideoxyhydroxide Graphite stage(A) storage(B) (%) 15 95  5 5 105  93 89 1690 10 5 113 104 92 17 80 20 5 124 109 88 18 50 50 5 144 114 79 19 20 805 150 105 70 20 10 90 5 157  86 55(Cut-off voltage 0.9 V)

As is clear from Tables 2 to 4, regardless of the content of nickeloxyhydroxide, the discharge performances after high temperature storageof the batteries comprising manganese dioxide having a potential of 270mV or higher were improved in comparison with the batteries comprisingmanganese dioxide having a potential lower than 270 mV.

The improvements in storage characteristics were particularly remarkablewhen the content of manganese dioxide with respect to the total amountof manganese dioxide and nickel oxyhydroxide was from 20 to 90 wt % andthe content of nickel oxyhydroxide was from 10 to 80 wt %.

It is noted that as the method of heightening the potential ofelectrolytic manganese dioxide, other methods than the method describedin the examples of the present invention may be employed.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to suppressself-discharge reaction of an alkaline battery comprising manganesedioxide and nickel oxyhydroxide as active materials and retain heavyload discharge performance of the alkaline battery even after storage.

1. An alkaline battery comprising: a positive electrode mixturecomprising manganese dioxide and nickel oxyhydroxide as activematerials; a negative electrode comprising zinc as an active material;and an alkaline electrolyte, characterized in that the potential of saidmanganese dioxide relative to a mercury/mercury oxide electrode in apotassium hydroxide aqueous solution having a KOH concentration of 40 wt% is 270 mV or higher.
 2. The alkaline battery in accordance with claim1, wherein with respect to the total amount of said manganese dioxideand said nickel oxyhydroxide, the content of said manganese dioxide isfrom 20 to 90 wt % and the content of said nickel oxyhydroxide is from10 to 80 wt %.
 3. The alkaline battery in accordance with claim 1,wherein said manganese dioxide is electrolytic manganese dioxide whosepotential is heightened by cleaning with an aqueous solution of sulfuricacid.
 4. The alkaline battery in accordance with claim 3, wherein theconcentration of sulfuric acid in said aqueous solution of sulfuric acidis 10 wt % or higher.